LTAO (Laser Tomography Adaptive Optics) encounters errors arising from both adaptive optics and laser tomography processes. This paper presents a comprehensive analysis of these errors under various operational conditions in laser tomography. Specifically, we examine the error variations with changes in atmospheric altitude, providing an in-depth error analysis associated with altitude fluctuations. Additionally, we analyze errors using single conjugate adaptive optics (SCAO) and the learn and apply (L&A) algorithm, highlighting the specific challenges and error metrics encountered in these methodologies.
This paper aims to analyze the characteristics of laser beam propagation from ground to space through atmospheric turbulence at the Gochang SLR Observatory in South Korea. To address issues such as energy loss and pointing errors caused by atmospheric turbulence in various applications like laser optical communication, satellite laser ranging (SLR), adaptive optics systems, and laser energy transmission, we systematically analyzed how laser beam parameters are affected by atmospheric conditions. The study utilizes a 100-watt-class laser to simulate beam propagation up to 100 km altitude, focusing on parameters such as laser wavelength, beam size (diameter), beam jitter, and beam quality (M²) and their impact on long-exposure and short-exposure beam radius. Key findings reveal that beam jitter is the most influential parameter for the long-exposure radius, while beam quality (M²) significantly affects the short-exposure radius. These findings provide critical guidance for optimizing laser system performance under varying atmospheric conditions, potentially contributing to advancements in laser communication and weapon systems.
Stray light, the unwanted light within an imaging system, can severely degrade the image quality of LWIR (Long-Wave Infrared) cameras, especially when it originates from external sources such as the sun or thermal emissions from internal components. While the Point Source Transmittance (PST) index measures an optical system's ability to mitigate stray light, it does not fully capture the extent of image degradation that results. In our study, we quantified image quality degradation by converting the distribution of stray light at the detector into digital grayscale levels. Based on this analysis, we conducted an optimal design that comprehensively accounts for both external and internal scattered light.
We developed SLODAR (SLOpe Detection And Ranging) in order to measure atmospheric characteristics in Geochang, Korea for the purpose of utilizing adaptive optics technology. In order to operate adaptive optics, it is necessary to measure the Fried parameter (r0) and Refractive index structure constant (Cn2) of the corresponding region, which can be measured through atmospheric characteristics measuring module. SLODAR mounts two wavefront sensors on a telescope and directs a binary star towards each sensor using a prism mirror. Then it measures the slope by detecting the focus shift caused by the atmospheric disturbance using the binary star. We attached z-axis motors to SLODAR and Prism to ensure that the focal position would always be the same even if the separation angle of the binary star changed. SLODAR can measure atmospheric characteristics at various altitudes, depending on the separation angle of the binary star. It is divided into wide mode (measured altitude about 400 ~ 700 m) and narrow mode (measured altitude about 5 ~ 9 km). We obtained r0 and Cn2 through Auto-Covariance and Cross-Covariance, respectively, from the measured slope. Currently, we are measuring the annual average r0 Cn2 in Geochang, Korea.
We present an implementation of tomography using MATLAB, specifically focusing on the Learn & Apply technique. The primary objective of our study is to assess the level of reconstruction errors encountered in tomography. We compare the results obtained from our implementation with the Object-Oriented, Matlab and Adaptive Optics (OOMAO) code, aiming to quantify the discrepancies between the two approaches. Furthermore, we investigate the impact of laser guide star (LGS) altitude variations on the reconstruction errors. By simulating tomographic reconstructions under different LGS altitudes, we analyze the variations in the error values and their relation to the atmospheric conditions. Additionally, we examine the influence of LGS divergence angle on the reconstruction errors, considering various divergence angles in our simulations. Afterwards, we explore the effect of increasing the number of layers used in the learning process on the simulation time.
The conventional on-axis reflective systems suffer from a diffraction effect on the Point Spread Function (PSF) due to the secondary mirror obscuration. Meanwhile, the unobscured off-axis reflective systems’ imaging performance may be impacted by linear astigmatism aberration. The Linear Astigmatism Free-Three Mirror System (LAF-TMS) is a confocal off-axis reflective system that eliminates linear astigmatism and enables a wide Field of View (FoV). We present an enhanced design of LAF-TMS, called ”wide-wide”, which has an aperture of D=40mm, an effective focal length of f=75mm, and a wide FoV of 8.25°(Horizontal) × 6.21°(V ertical) combined with a wide spectral bandwidth capability suitable for Unmanned Aerial Vehicle (UAV) applications. To evaluate the performance of this compact and fast optical system design, we use the Photon Simulator (PhoSim) to model physically accurate PSF under different conditions of the mirror surface, mechanical environment, and atmosphere. As a benchmark, we compare and analyze the PhoSim PSF results with other ray tracing software such as Zemax and CodeV. Additionally, PhoSim is capable of simulating infrared spectral imaging cases with a user-defined Spectral Energy Distribution (SED), intensity, and emissivity of each pixel. The comprehensive simulation results demonstrate the high performance of the LAF-TMS with a wide-wide FoV and multispectral capabilities.
The nature of optical diffraction limits the spatial resolution of a space-borne optical sensor to the diffraction limit. Using the Rayleigh criterion, we match the Airy Diameter of the optical sensor to the pitch size of a selected imaging detector, or vice versa. Since there is little choice when selecting space-proven imaging detectors, conventional approaches have few options for payload optimization in terms of compactness or volume. In this paper, we first propose an innovative concept of volume sharing multi-aperture payloads to achieve ground spatial resolutions several times higher than conventional approaches. With this approach, we can achieve 1 m ground sampling distance (GSD) at 500 km altitude with a small satellite of less than 100U volume.
KEYWORDS: Defect detection, Optical microscopy, Near infrared, 3D modeling, 3D metrology, Data modeling, 3D acquisition, Wafer-level optics, Semiconductors
Through-focus scanning optical microscopy (TSOM) is a model-based optical metrology method that involves the scanning of a target through the focus of an optical microscope. Nanometer scale sensitive information is then extracted by matching the target TSOM data/image to reference TSOM data/images that are either experimentally or computationally collected. The nanometer sensitivity was previously confirmed by several theoretical and optical implementations. However, these studies all involved application to wafer patterns on the top surface. The present study extends the TSOM method to subsurface defect detection and classification without destruction, which becomes extremely important due to increasingly widely employed 3D semiconductor technologies. First, we apply a near-infrared (NIR) beam as illumination light in order to allow defect identification over the entire device depth. In addition, we adopt a model-less TSOM approach since the construction of a TSOM reference database for 3D pattern structures such as 3D NAND flash memory is hardly practical. We therefore employ a comparative TSOM method in which a TSOM data cube/image is compared with an image of an adjacent die or that of a “golden” die known to be defect free. We report the results of the first application of this method to an Intel 3D NAND flash and show that substantial subsurface defects are detected and classified.
Metrology and inspection (MI) processes are established at critical points of the semiconductor manufacturing process in order to maintain a certain yield and also provide information needed for future processes improvements. Typically, the inspection consists of dark-field (DF) inspection and SEM review/classification processes. An optical DF microscopy system (or inspection tool) first detects particles or pattern defects on wafers and obtains their position coordinates. However, due to its limited optical resolution, the DF system is not widely applied in the review process, which requires higher resolution images of the detected defects such as those provided by scanning electron microscopy (SEM) but with a sacrifice of throughput. We propose an innovative idea of applying two (or three) dark-field microscopy images for intermediate defect classification and size estimation under optical resolution. The proposed method utilizes the angular scattering distribution from a defect that is in the Mie scattering domain, which varies depending on both the beam and defect properties (wavelength, polarization, incident angle; shape, size, complex refractive index). It captures three darkfield images of the same wafer by three inter-changeable objectives with different magnification and numerical aperture (NA) values under identical side illumination conditions. We estimate the defect types and sizes simply by investing three measurements. We demonstrated this proposed method to classify and estimate the defect size down to ~ 80nm by an existing UV inspection tool with three DF imaging modes; 1) M15 mode, sampling stance = 150 nm, NA =0.6, 2) M25 mode, sampling distance = 250 nm, NA=0.36, and 3) M40 mode, sampling distance =400 nm, and NA =0.23. We demonstrated its feasibility by an independent SEM measurement of the detected defects.
Through-focus optical microscopy (TSOM) with nanometer-scale lateral and vertical sensitivity levels matching those of scanning electron microscopy has been demonstrated to be useful both for 3D inspections and metrology assessments. In 2014, funded by two private companies (Nextin/Samsung Electronics) and the Korea Evaluation Institute of Industrial Technology (KEIT), a research team from four universities in South Korea set out to investigate core technologies for developing in-line TSOM inspection and metrology tools, with the respective teams focusing on optics implementation, defect inspection, computer simulation and high-speed metrology matching. We initially confirmed the reported validity of the TSOM operation through a computer simulation, after which we implemented the TSOM operation by throughfocus scanning of existing UV (355nm) and IR (800nm) inspection tools. These tools have an identical sampling distance of 150 nm but have different resolving distances (310 and 810 nm, respectively). We initially experienced some improvement in the defect inspection sensitivity level over TSV (through-silicon via) samples with ~ 6.6 μm diameters. However, during the experiment, we noted sensitivity and instability issues when attempting to acquire TSOM images. As TSOM 3D information is indirectly extracted by differentiating a target TSOM image from reference TSOM images, any instability or mismatch in imaging conditions can result in measurement errors. As a remedy to such a situation, we proposed the application of adaptive optics to the TSOM operation and developed a closed-loop system with a tip/tilt mirror and a Shack-Hartmann sensor on an optical bench. We were able to keep the plane position within in RMS 0.4 pixel by actively compensating for any position instability which arose during the TSOM scanning process along the optical axis. Currently, we are also developing another TSOM tool with a deformable mirror instead of a tip/tilt mirror, in which case we will not require any mechanical scanning.
We report an innovative simple alignment method for a VNIR spectrometer in the wavelength region of 400–900 nm; this device is later combined with fore-optics (a telescope) to form a f/2.5 hyperspectral imaging spectrometer with a field of view of ±7.68°. The detector at the final image plane is a 640×480 charge-coupled device with a 24 μm pixel size. We first assembled the fore-optics and the spectrometer separately and then combined them via a slit co-located on the image plane of the fore-optics and the object plane of the spectrometer. The spectrometer was assembled in three steps. In the initial step, the optics was simply assembled with an optical axis guiding He-Ne laser. In the second step, we located a pin-hole on the slit plane and a Shack-Hartmann sensor on the detector plane. The wavefront errors over the full field were scanned simply by moving the point source along the slit direction while the Shack-Hartmann sensor was constantly conjugated to the pin-hole position by a motorized stage. Optimal alignment was then performed based on the reverse sensitivity method. In the final stage, the pin-hole and the Shack-Hartmann sensor were exchanged with an equispaced 10 pin-hole slit called a field identifier and a detector. The light source was also changed from the laser (single wavelength source) to a krypton lamp (discrete multi-wavelength source). We were then easily able to calculate the distortion and keystone on the detector plane without any scanning or moving optical components; rather, we merely calculated the spectral centroids of the 10 pin-holes on the detector. We then tuned the clocking angles of the convex grating and the detector to minimize the distortion and keystone. The final assembly was tested and found to have an RMS WFE < 90 nm over the entire field of view, a keystone of 0.08 pixels, a smile of 1.13 pixels and a spectral resolution of 4.32 nm.
Through-Focus Optical Microscopy (TSOM), with nanometer scale lateral and vertical sensitivity matching those of scanning electron microscopy, has been demonstrated to be utilized for 3D inspection and metrology. There have been sensitivity and instability issues in acquiring through-focus images because TSOM 3D information is indirectly extracted by differentiating a target TSOM image from reference TSOM images. This paper first reports on the optical axis instability that occurs during the scanning process of TSOM when implemented in an existing patterned wafer inspection tool by moving the wafer plane; this is followed by quantitative confirmation of the optical/mechanical instability using a new TSOM tool on an optical bench with a Shack-Hartmann wavefront sensor and a tip/tilt sensor. Then, this paper proposes two tip/tilt compensated TSOM optical acquisition methods that can be applied with adaptive optics. The first method simply adopts a tip/tilt mirror with a quad cell in a simple closed loop, while the second method adopts a highorder deformable mirror with a Shack-Hartmann sensor. The second method is able to correct high-order residual aberrations as well as to perform through-focus scanning without z-axis movement, while the first method is easier to implement in pre-existing wafer inspection systems with only minor modification.
Currently we are developing a 10 cm silicon carbide (SiC) deformable mirror with 37 actuators operating at 500 Hz. The deformable mirror will be applied in a 1.5 m telescope. An adaptive optics system capability for the deformable mirror was simulated and performance was predicted based on the Kolmogorov atmospheric turbulence model. However, in order to confirm the predictions, a closed-loop adaptive optics system was constructed with the insertion of an atmospheric turbulence simulator consisting of two point sources, a Boston deformable mirror, and double random phase plates. In order to simulate a binary star, the two point sources are mounted on 3-axis micron meter stages and are optically merged into a single beam path by a beam splitter cube. The light intensity of each source is adjustable to a corresponding stellar magnitude, while the angular separation can be precisely adjusted by moving the 3-axis stages. The atmospheric disturbance is generated by shaping the Boston deformable mirror and also by rotating the double phase plates. The Fried parameter of the generated the atmospheric disturbance corresponds to an area from 7 to 15 cm at 500 nm at the telescope pupil plane, which represents typical seeing conditions at the Bohyun observatory, South Korea.
This paper describes about the new design method for hyper-spectral Imaging spectrometers utilizing convex grating. Hyper-spectral imaging systems are power tools in the field of remote sensing. HSI systems collect at least 100 spectral bands of 10~20 nm width. Because the spectral signature is different and induced unique for each material, it should be possible to discriminate between one material and another based on difference in spectral signature of material.
I mathematically analyzed parameters for the intellectual initial design. Main concept of this is the derivative of "ring of minimum aberration without vignetting". This work is a kind of analytical design of an Offner imaging spectrometer.
Also, several experiment methods will be contrived to evaluate the performance of imaging spectrometer.
An airborne sensor is developed for remote sensing on an unmanned aerial vehicle (UAV). The sensor is an optical
payload for an eletro-optical/infrared (EO/IR) dual band camera that combines visible and IR imaging capabilities in a
compact and lightweight manner. It adopts a Ritchey-Chrétien telescope for the common front end optics with several
relay optics that divide and deliver EO and IR bands to a charge-coupled-device (CCD) and an IR detector, respectively.
For the easy assemble of such a complicated optics, a computer-aided alignment program (herein called simulator) is
developed. The simulator first estimates the details of the misalignments such as locations, types, and amounts from the
test results such as modulation transfer function (MTF), Zernike polynomial coefficients, and RMS wavefront errors at
different field positions. Then it recommends the compensator movement(s) with the estimated optical performance. The
simulator is coded on Matlab with the hidden connection to optical analysis/design software Zemax. By interfacing
ZEMAX and MATLAB, the GUI-based alignment simulator, will help even those not familiar with the two programs to
obtain accurate results more easily and quickly.
We introduce a design of an Offner imaging spectrograph with its performance and tolerancing results. It is a traditional Offner spectrograph employing two concave mirrors and one convex reflective grating for dispersing light in the SWIR band (900~1700 nm). The optical system uses 25um-pitch pixels for the detector and the goal spectral sampling is 3.2nm. Its performance is analyzed in terms of MTFs, spot diagrams, and distortions – keystone and smile. This design focuses on the yaw(beta-tilt) sensitivity of the tertiary mirror as the compensator hence is expected to act as a performance-improving breakthrough for the entire system as the inverse sensitivity confirms it is the most sensitive component. The procedure of the inverse sensitivity evaluation is explained, and then budgeting the tolerances for each element for the practical production is described.
In recent years, high performance visible and IR cameras have been used widely for tactical airborne reconnaissance.
The process improvement for efficient discrimination and analysis of complex target information from active battlefields
requires for simultaneous multi-band measurement from airborne platforms at various altitudes. We report a new dual
band airborne camera designed for simultaneous registration of both visible and IR imagery from mid-altitude ranges.
The camera design uses a common front end optical telescope of around 0.3m in entrance aperture and several relay
optical sub-systems capable of delivering both high spatial resolution visible and IR images to the detectors. The camera
design is benefited from the use of several optical channels packaged in a compact space and the associated freedom to
choose between wide (~3 degrees) and narrow (~1 degree) field of view. In order to investigate both imaging and
radiometric performances of the camera, we generated an array of target scenes with optical properties such as reflection,
refraction, scattering, transmission and emission. We then combined the target scenes and the camera optical system into
the integrated ray tracing simulation environment utilizing Monte Carlo computation technique. Taking realistic
atmospheric radiative transfer characteristics into account, both imaging and radiometric performances were then
investigated. The simulation results demonstrate successfully that the camera design satisfies NIIRS 7 detection criterion.
The camera concept, details of performance simulation computation, the resulting performances are discussed together
with future development plan.
STSAT3, a ~150 kg micro satellite, is the third experimental microsatellite of the STSAT series designated in the Long-
Term Plan for Korea's Space Development by the Ministry of Education, Science and Technology of Korea. STSAT3 is
being developed for launch into a sun-synchronous orbit of 700 km altitude by the end of 2010. A compact imaging
spectrometer (COMIS) is a secondary payload of STSAT3 that will be employed for environmental monitoring, mainly
over the Korean peninsula. COMIS was inspired by the success of CHRIS, a previous PROBA payload. The chief
function of COMIS is to image the Earth's surface with ground sampling distances of 30m or less at 18~62 spectral
bands (4.0~1.05μm) for nadir observation at 700km altitude. COMIS, as its name implies, is very compact in volume,
mass, and power. The total mass including optics, housing, and electronics is about 4.3kg and the average power per
orbit is less than 5 watt. This paper reports on the prototype development of COMIS.
A LED light coupler is designed with rigorous geometrical optics analysis for coupling light from a mini side white LED
of 0.6mm height, the thinnest commercially available LED, into to a very thin (~0.3mm) light guide on the edge side.
The ultra thin thickness of the light guide limits the application of conventional LED coupling due to short coupling
efficiency. Furthermore the polychromatic spectral band of the LED excludes the use of diffractive gratings which were
proposed for a nearly monochromatic LED coupling to a thin light guide. The coupler is essentially comprised of a
collimator and a concentrator lens. The coupler first collimates the light from the LED and then concentrates the
collimated light into the light guide. The exit surface of the coupler is additionally textured to have micro patterns in
order to control the radiance profile. The coupling efficiency is improved from 50%~60% without any couplers to 86%
with the proposed LED coupler.
This paper proposes a new, simple, high-speed wafer-inspection method using a knife-edge approach. This method
basically corresponds to a bright-field microscope except the light reflected from the wafer surface is partially cut by a
knife-edge, which increases edge contrast in the cutting direction. In addition, the knife-edge test does not depend on the
light wavelength and thus the light can have any wavelength as far as it is reflected by the wafer surface. Therefore, the
knife-edge test, which uses visible light, can be employed simultaneously with a looking through inspection technique,
which uses infra-red, with a single white-light source. The knife-edge method in tandem with a looking through
inspection is implemented at a laboratory level and preliminary experimental results demonstrating edge enhancement
are presented.
STSAT-3, a ~150kg micro satellite, is the third experimental microsatellite of the STSAT series designated in the Long-
Term Plan for Korea's Space Development by the Ministry of Science and Technology of Korea. The STSAT-3 satellite
was initiated in October 2006 and will be launched into a lower sun-synchronous earth orbit (~ 700km) in 2010. This
paper presents a brief introduction of STSAT-3 and also introduces its secondary payload, i.e. COMIS, a compact
imaging spectrometer, which was inspired by the success of CHRIS, a previous PROBA payload. COMIS takes hyperspectral
images of 30m/60m ground sampling distance over a 30km swath width. The number of bands is selectable
among 18 or 62. COMIS takes hyper-spectral images in two different modes: a) Pushbroom and b) multi-directional
observation. The payload will be used for environmental monitoring, such as in-land water quality monitoring of Paldang
Lake located next to Seoul, the capital of South Korea.
Wavefront sensing using a Shack-Hartmann sensor has been widely used for estimating wavefront errors or distortions. The sensor combines the local slopes, which are estimated from the centroids of each lenslet images, to give the overall wavefront reconstruction. It was previously shown that the pupil-plane irradiance profile effects on the centroid estimation. Furthermore, a previous study reported that the reconstructed wavefront from a planar wavefront with a Gaussian pupil irradiance profile contain large focus and spherical aberration terms when there is a focus error. However, it has not been reported yet how serious the pupil irradiance profiles, which can be occurred in practical applications, effects on the sensing errors. This paper considered two cases when the irradiance profiles are not uniform: 1) when the light source is Gaussian and 2) when there is a partial interference due to a double reflection by a beam splitting element. The images formed by a Shack-Hartmann sensor were simulated through fast Fourier transform and were then supposed to be detected by a noiseless CCD camera. The simulations found that sensing errors, due to the Gaussian irradiance profile and the partial interference, were found to be smaller than λ/50 which can be ignored in most practical cases where the reference and test beams have the same irradiance profiles.
Lightweight mirrors experience optical image degradation due to mechanical loadings such as self-weight, polishing pressure, and vibration. Optical surface deformation of a lightweight primary mirror is an important factor that affects optical performance. We use topology optimization to design a lightweight primary mirror under self-weight and polishing pressure. For the optimization, we used a 3-D model of the mirror and based our calculations on the rms surface error of the mirror as an objective function constrained by the maximum weight of the mirror. In the first example of topology optimization, we consider the mirror's self-weight loading. In the second example, we include the polishing pressure. We present the results of the optimized design topology for the mirror with various mass constraints. To examine the optimal design results, we manufacture a prototype of the mirror.
The performance of ground telescopes is limited by atmospheric distortion. Nowadays all large ground telescopes adopt adaptive optics for overcoming this limitation. Sending space telescopes outside the troublesome atmosphere might be a natural solution to overcome the atmospheric distortion. However, the cost of development and launch, and the size of launch fairing severally limit this option. Inflatable optics is major candidate for overcoming these technical and budget limits. In this study, we performed thickness optimizations of a membrane mirror for the mirror to be parabolic. Our optimization showed that this optimized mirror is still not good enough for visible observation. However, with limiting the effective optical surface area, the mirror was demonstrated to be used as a primary mirror in infrared bands. In addition, the wavefront errors are also shown to main contributors: piston, defocus and spherical aberrations. Adapting an adaptive secondary mirror with 19 actuators, which has been developed for ground telescopes, could remove the major wavefront errors. Therefore, combining an inflatable primary mirror and an adaptive secondary mirror can be a candidate for future large space telescopes.
We are performing a research on the application of an adaptive optics system to upgrade the beam quality of a laser. We consider the adaptive optics system to consist of a bimorph mirror, a Shack-Hartmann sensor and a control system. Since the laser beam can be circular, annulus or square/rectangle, we predicted the performance of a circular bimorph deformable for each different beam shapes. We selected a bimorph mirror of 120 mm clear aperture with 31 actuators as a design candidate. Firstly we found that the fitting ability of the bimorph mirror for a circular/annulus beam can significantly improve by limiting the correctable area to an annulus of 100mm outer diameter and 20mm inner diameter, which our square laser beam fits into. This study shows that the bimorph mirror, which might be considered as a modal controller, can compensate the square lower order aberrations with fitting abilities larger than 0.95 for tilt, defocus, coma and astigmatism, and 0.82 for spherical aberration. Finally we concluded that the circular bimorph mirror is perfectly okay for square laser beam compensating.
The Shack-Hartmann or Hartmann-Shack wavefront sensors are particular forms of the Hartmann sensor and are the most commonly used in adaptive optics. The traditional Hartmann screen in the pupil is replaced by an array of small lenslets at a conjugate to an image of the pupil or deformable mirror. Each lenslet forms an independent image of the incoming wavefront. Shifts in the positions of these images can be shown by simple geometric optics to be proportional to the mean wavefront gradient over each lenslet. The measurement range or dynamic range of a conventional Shack-Hartmann sensor is normally limited by the sub-aperture size of the detector plane, in which each spot should remain. In order to overcome this restriction, several methods, such as modified unwrapped algorithm or a spatial-light modulator as a shutter, have been proposed. We first simulated the image forming of a conventional Shack-Hartmann wavefront sensor for highly aberrated spherical aberrations through computer simulation, which confirmed that the well-known effect that high aberration shifts the spots outside its conventionally detectable area. We first develop a computer program that simulates Shack-Hartmann’s image forming and we show that simple defocusing the CCD plane can allocate the out-boundary spots to initial or reference positions, which results in the increase of the dynamic range of the Shack-Hartmann sensor.
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